Record Heat: A Review of 2023 and 2024
Scorched Summers
Averaged across the globe, 2023 was 2.6oF warmer than the period (1850-1900) used as a pre-industrial baseline.[1] Winter months are warming about twice as fast as summers, but the increasing temperature trend is consistent across all seasons. NASA has found recent global summer temperatures now average 2 degrees warmer than they were around mid-century (1951-1980), and in the United States they are now 2.5oF warmer. Even absolute (non-averaged) temperature records were broken by wide margins; September 2023 was “massively” (in climatological terms) warmer (by 1.2oF) than the previous record September set in 2002.[2] As we’ve explored in previous posts, such seemingly small increases in average temperature have large impacts on the frequency and intensity of extreme events like heatwaves.
NASA Image. “Temperature anomaly” refers to how much temperature differs from the long-term average. Note the temperature scale is in Celsius (1oC equals 1.8oF).
Last year also broke records for humidity, a critical factor in determining how much heat humans can tolerate (see the Thermophysiology 102 for how the combination of heat and humidity affects human thermal limits). In their annual “State of the Climate” report, the American Meteorological Society notes the amount of water vapor in the atmosphere was the highest ever recorded in 2023. The combined effects of 2023’s heat and humidity created conditions for humid-heat indices to set new records and, by some accounts, these indices have doubled in intensity. Increasing humid-heat indices are relevant for understanding how fast recent extreme temperatures are approaching the theoretical limits of human comfort and safety.[3]
Despite record setting heat in 2023, summer temperatures this year were even hotter. August 2024 was the 15th consecutive month of record high monthly temperatures – and the warmest in NOAA’s 175-year record. This August heat is part of ongoing trend, with the last eleven consecutive summers (June-August) being the warmest on record.
Temperatures are shown as June-August average anomalies. Note the temperature scale is in Celsius (1.24oC is 2.23oF).
Why The Recent Heat?
Not all record-breaking heat over the last couple years can be attributed to climate change. El Nino, the “warm” phase of a natural cycling of hot and cold ocean temperatures in the Pacific, correlates to a global temperature increase. To (over)simplify, during El Nino periods heat stored in the ocean is released into the atmosphere. 2023-2024 experienced strong El Nino conditions, partly contributing to recent record-setting warm temperatures around the globe. The El Nino-Southern Oscillation (ENSO), which includes El Nino and it’s opposite (cooler) phase, La Nina, are one of the most important drivers of natural global temperature variability. It’s estimated 2023-2024 El Nino conditions added about half a degree Fahrenheit to recent temperature increases – a non-trivial amount, but only responsible for about 20% of recent temperature increases.
Two other non-climate change factors have been explored for links to recent temperature increases. A recent reduction in atmospheric aerosols has also likely contributed to recent record warmth. Aerosols, which are small particles in the air that generally reflect sunlight and promote sun-blocking cloud formation, are a byproduct of (among other things) burning the type of fuel used by ocean-crossing container ships. Recently implemented regulations designed to reduce pollution (in particular, sulfur dioxide) have decreased the amount of aerosols released by international shipping by about 80% compared to 2020. However, the effect of aerosol reduction on global temperature increase is also small, contributing to, at most, 20% of 2023’s record heat.
Assuming the higher estimates are correct (there’s always some uncertainty in these sort of studies), El Nino and aerosol reduction combined may have contributed to not quite half of the global record heat in 2023. A third potential culprit for recent warming is the early 2022 eruption of the Hunga Tongo volcano. Most volcanic eruptions decrease global temperatures by releasing huge amount of aerosols, but this eruption– more powerful than the largest nuclear bomb ever detonated- occurred mostly under the ocean, sending enough water to fill 58,000 Olympic swimming pools into the stratosphere. Atmospheric water vapor is a greenhouse gas [4], warming the planet, but scientists have concluded that despite its size, this eruption did not contribute to temperature increases observed in 2023 and 2024.
Hunga Tonga explosion, January 15th, 2022. Satellite image from the NASA visible earth catalogue.
Why it Matters
Summers in 2023 and 2024 smashed previous temperature records. Through 2022, the long-term global warming average has been about 0.27-0.36oF per decade. In the last two years, temperature increases far exceeded this trend, increasing about 0.5oF since just 2022. This sort of rapid increase keeps climate scientists up at night. While daily temperatures have far more variability (think the difference in temperature between night and day), increases in global average temperature measure excess energy retained by the planet. Warming the globe by half a degree requires retaining an incredible amount of excess energy, measured in terms of zettajoules. “Zettajoules” is a meaningless concept for most of us, so it needs context. 1 Zettajoule is 1,000 Exajoules. “Exajoules” is also meaningless (sorry…). For comparison, the entire world’s energy production in 2023 was around 620 Exajoules (all this to demonstrate, recent globally averaged temperature change requires huge amounts of heat are being added or retained in the atmosphere.)
It's unknown if the rate of temperature increase over the last couple years will keep surging or fall back to a slower pace; scientists offer conflicting answers. Some studies find that summer 2023 temperatures, while much higher than average, don’t yet prove the pace of global warming has increased since 1970 (more properly, these studies claim the recent uptick is within the limits of expected statistical variability). Others find temperatures in 2023 and 2024 spiked so much that projections relying on previous observations might no longer be useful for anticipating the future.
Understanding what to expect in the future is not just an academic pursuit. As we explored previously, small increases in average temperature have large implication for the frequency and severity of heatwaves and record hot temperatures, leading to longer periods of more extreme heat. Consider Death Valley, CA, one of the hottest places on Earth, which reached 125oF for nine consecutive days in July 2024 (and a maximum of 129.3oF on July 7th), triggering multi-day power outages within the park. Summer heat index values above 150oF – approaching the thermal limit humans can survive for more than a few hours [5] – have recently been recorded around the Persian Gulf.
Small increases in global temperatures also create conditions of extreme heat outside of normal summer months. This year, Long Beach, CA, set a daily temperature record of 109oF on September 6th, 25oF warmer than normal for that day of the year. Florida and Gulf Coast States experienced a Spring heatwave in early May, with the heat index topping 112oF. Such early season heatwaves are particularly dangerous since people have not had weeks to more slowly acclimatize to warming late spring and early summer temperatures. At the opposite end of the calendar, Western U.S. States issues triple-digit heatwave warnings in October, well into the Fall and a time when many people have already put out Halloween decorations.
Periods of extended heat have economic impacts for the unprepared. The Federal Reserve Bank in Dallas estimates that Texas’ annual nominal GDP growth in 2023 may have been reduced by 1% due to that year’s extreme heat – or around $24 Billion.[6] The costliest disaster in the U.S during 2023- estimated at $14.5 Billion- was a combination drought and heatwave across the South and Midwest that affected agricultural production and was implicated in 247 deaths. Economic impacts of heat and drought extend beyond agriculture in ways that aren’t immediately obvious. At one point in 2023, low water in the Mississippi River brought more than 2,000 river barges to a halt, snarling river-dependent supply chains for grain, fertilizer, aggregate, and chemicals.
Despite huge impacts, an economic assessment of extreme heat doesn’t describe the full impact. Extreme heat also challenges the men and women guarding our borders. Customs and Border Patrol agents operating along the Southern border started getting issued heat stress kits in 2022. Fatal cases of heat stroke and dehydration aren’t uncommon among migrant groups crossing the border illegally, something the Border Patrol’s Search, Trauma, and Rescue Team (BORSTAR) knows all too well. BORSTAR members combine law enforcements skills with advanced medical training, and most summer medical emergenciesare heat-related. Agents, too, are affected by the heat, as demonstrated by the tragic deaths of agents Eric Cabral and Johan Mordan. Federal efforts to address agent heat stress include bipartisan bills in both the Senate and House aiming to adapt policy and equipment to be more resilient despite increasing temperatures.
Image: BORSTAR agents provide medical attention to an individual experiencing heat illness near McAllen, TX. Photo credit Mani Albrecht / CBP Visual Communications Division.
Extreme heat in 2023 and 2024 provides us with a strong warning – get ready. The U.S. is better positioned than most to respond to a warmer future, thanks in large part to the innovative and adaptive spirit of the Country. While temperature trends may not continue to rise as sharply as they have in the last couple years, the direction is still certain to be upward. It’s probable the coolest year we’ve experienced in our lifetimes is well behind us.
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About the author: Erik is a doctoral candidate at Duke University where he conducts research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).
[1] Sufficiently accurate, consistent, and widespread observations of global temperature aren’t available much earlier than 1850, although some research efforts claim (with good justification) to have accurate temperature records back to about 1750, albeit only over land and for certain regions.
[2] If the numbers thrown around in this paragraph appear to contradict, remember that global temperature includes the 70% of the earth covered by ocean, which moderate temperature change. Land surface temperature is more dynamic (warming and cooling faster than oceans) and depends on factors such as land use, topography, and vegetation cover, so different regions experience different rates of warming. Since land surfaces are where we live, land surface temperature measurement is more relevant.
[3] Their report for 2024 won’t be released until next year. The AMS defines “intensity” here as the wet bulb temperature anomaly, similar to how the temperature is described in the first figure.
[4] In fact, water vapor contributes to about half of Earth’s greenhouse gas effect. Without water in our atmosphere, Earth would be far too cold for life as we know it. Atmospheric water vapor concentration is relatively steady and constantly recycling (think rainfall), so it does not contribute to long term climate change.
[5] We explored human thermal limits using the wet bulb index in this article. Although the heat index also factors in humidity, it is a completely different thermal index – wet bulb temperature and heat index should not be directly compared when considering human thermal tolerance. A wet bulb temperature of 150oF would be well beyond lethal for humans.
[6] The 1% GDP decrease figure is taken straight from the Dallas Fed’s webpage, however, the Dallas Fed notes that “other calculations suggest a somewhat lesser impact of nearly $10 Billion”. That’s still a lot of money.